ublk: remove io argument from ublk_auto_buf_reg_fallback()

The argument has been unused since the function was added, so remove it.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250521160720.1893326-1-csander@purestorage.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>